This invention relates to testing of an optical fiber coupler for use in wavelength division multiplexing.
Mach-Zehnder-type interferometers (xe2x80x9cMZIsxe2x80x9d) have recently been applied to dense wavelength division multiplexing (DWDM), which may be defined as a WDM system in which a single mode optical fiber is altered to carry at least 16 signals with spaced apart carrier frequencies. WDM and DWDM systems are discussed in the context of increasing signal density requirements by J. Hecht in xe2x80x9cWavelength Division Multiplexingxe2x80x9d, Technology Review, March-April 1999, pp. 73-78., and by P. Fairley in xe2x80x9cThe Microphotonics Revolutionxe2x80x9d, Technology Review, July-August 2000, pp. 38-44. An MZI DWDM is conventionally implemented by connecting two 3 dB couplers having different lengths to two interferometer arms to create a controllable phase difference for the two arms. A common method for connecting the two arms is splicing, which offers an advantage of using qualified and well tested 3 dB couplers to create the same optical isolation at the output terminals. Major disadvantages of the splicing approach are (1) an uncertainty in signal insertion loss and (2) a tendency for a fiber to break near the splicing location.
What is needed is an approach for fabricating, connecting and testing the arms of an interferometer device that avoids or minimizes these problems and allows use of indirect standards to evaluate the connection. Preferably, the approach should work with already-available test equipment and should provide an unambiguous and quantitative test for acceptance. Preferably, the approach should be usable for two or more couplers in series.
These needs are met by the invention, which provides first and second 3 dB optical couplers, spaced apart by 35-45 mm, for two fibers. A fiber complex including a segment from each fiber is formed, using heating, twisting and elongation. The fiber complex controllably couples light from one fiber into the other fiber and is positioned between the first and second couplers. Coupling in the first fiber coupler and fiber complex may be tested in isolation, using a common light beam propagating in the two fibers. Fiber segments in the second coupler are elongated, and successive maximum and minimum values of an interferometric variable are measured by an optical spectrum analyzer as elongation proceeds. If (1) successive maxima lie within an envelope of selected small width, (2) the difference in amplitudes between successive maximum and minimum values is at least equal to a selected threshold difference and (3) pit-to-pit spacing (or peak-to-peak spacing) is close to a specified channel spacing, the combination is accepted for use in an interferometer.